feat(home): Hero 动态粒子层（入场浮现后静止）

新增 hero-particle-field 组件作为 Hero L3 层：品牌红点缀、Canvas + rAF
仅在 ~600ms 入场窗运行后冻结为静态帧，遵守「无持续循环动画」动效铁律；
prefers-reduced-motion 退化为一次性静态帧。固定种子 PRNG 保证视觉快照可复现。
同步更新 home-content-v15 接入与单测、home 视觉基线。

Co-Authored-By: 阿睿 <workbuddy@tencent.com>

src/app/(marketing)/home-content-v15.test.tsx

@@ -98,6 +98,16 @@ describe('HomeContentV15 - 浅色 Hero（暗黑模式 token 友好）', () => {
     expect(hero.querySelector('img')).toBeNull();
   });
 
+  it('mounts the brand-red dynamic particle layer (entrance-only, no continuous loop)', () => {
+    render(<HomeContentV15 />);
+
+    const hero = screen.getByTestId('hero-section');
+    const particles = screen.getByTestId('hero-particle-field');
+    expect(particles.tagName).toBe('CANVAS');
+    expect(particles).toHaveAttribute('aria-hidden', 'true');
+    expect(hero.contains(particles)).toBe(true);
+  });
+
   it('keeps a single primary CTA and demotes the secondary action to a text link', () => {
     render(<HomeContentV15 />);